Position detection is conventionally performed by using two-phase sinusoidal wave signals (referred to as signals V.sub.a and V.sub.b hereinafter) having a 90.degree. -spatial phase difference therebetween, and a position detecting apparatus of this type is generally called an encoder.
A conventional encoder irradiates light emitted from, e.g., a light-emitting diode, to a rotary encoding plate (normally provided with a large number of slits (e.g., 2,000) at equal intervals on the periphery thereof) mounted on a given rotating shaft, whose rotating position is to be detected. The light passed through a slit formed in the rotary encoding plate and a slit formed in a fixed plate facing the rotary encoding plate is received by a light-receiving element (e.g., a photodiode) to be converted into a voltage, thereby obtaining, in accordance with rotation of the rotary encoding plate, sinusoidal wave signals V.sub.a and V.sub.b, one cycle of which corresponds to an interval between two adjacent slits (one pitch) formed in the rotary encoding plate.
In this case, in order to provide the 90.degree. -spatial phase difference between the two sinusoidal wave signals V.sub.a and V.sub.b, two slits, shifted by 1/4 of a pitch from an integer multiple of the slit interval (1 pitch) of the rotary encoding plate, are formed in the fixed slit plate, and the two sinusoidal wave signals V.sub.a and V.sub.b are obtained from the electrical signals generated in the light-receiving elements, each of which faces each of the above two slits (see FIG. l(a)).
The sinusoidal wave signals V.sub.a and V.sub.b, are input to comparators for generating output signals AP and BP, which go to a HIGH or a LOW level in accordance with positive or negative levels of the sinusoidal wave signals. A so-called positive pulse for sequentially counting up an up-down counter (meaning that the rotary encoding plate is rotated in a given direction, e.g., clockwise) is produced in the following states: .circle. when the output signal BP is at a HIGH level at the leading edge of the output signal AP; .circle. when the output signal AP is at a HIGH level at the trailing edge of the output signal BP; .circle. when the output signal BP is at a LOW level at the trailing edge of the output signal AP; and .circle. when the output signal AP is at a LOW level at the leading edge of the output signal BP (see FIG. 1(b)). On the other hand, a so-called negative pulse for sequentially counting down the up-down counter (meaning that the rotary encoding plate is rotated in the opposite direction, e.g., counterclockwise) is produced in the following states: .circle. when the output signal BP is at a LOW level at the 1 - edge of the output signal AP; .circle. when the output signal AP is at a LOW level at the trailing edge of the output signal BP; .circle. when the output signal BP is at a HIGH level at the trailing edge of the output signal AP; and .circle. when the output signal AP is at a HIGH level at the leading edge of the output signal BP (see FIG. 1(c)). In this manner, the rotational angle of the rotary encoding plate, i.e., a rotating position of the rotating shaft on which the rotary encoding plate is mounted, is detected (in some cases, the rotational angle of the rotary encoding plate is linearly developed, and the moving position of a linearly moving object is detected).
However, in this conventional position detecting method, only four pulses (the positive or negative pulses) for counting up or down the up-down counter are generated upon rotation of the rotary encoding plate during 1 cycle of the output signal AP or BP (corresponding to a 1-pitch interval of two adjacent slits in the rotary encoding plate), as described above. If 2,000 slits are formed in the periphery of the rotary encoding plate, the resolution of the rotational angle detection is only 1/(2,000.times.4)=1/8,000 rotation. In other words, upon rotation of the rotary encoding plate, only a detection resolution corresponding to 4 times of a divisor for each rotation of the rotary encoding plate, corresponding to the number of slits, can be obtained.